Treatment of metastatic melanoma with immune checkpoint inhibitors (ICI) has extended the life of many melanoma patients, but the vast majority of patient experience disease progression, prompting the need for alternate therapies. For ~50% of patients with BRAFmut tumors, treatment with BRAF and MEK inhibitors provide a good second-line treatment option. Unfortunately, there are few second-line options for the 25-30% of patients whose tumors harbor NRAS mutations. Since nearly 40% of all melanoma patients exhibit loss, mutation, or epigenetic silencing of the CDK4/6 regulator CDKN2A, we postulate that inhibition of CDK4/6 may induce response in NRASmut RBWT tumors with loss of CDKN2A. Because loss of CDKN2A also disrupts ARF, a suppressor of MDM2-mediated degradation of p53, it will be essential to also inhibit MDM2 to restore cell cycle control in p53WT melanoma. We have shown that NRASmut melanoma tumors with acquired resistance to ICI respond to co-treatment with a CDK4/6 inhibitor plus an MDM2, demonstrating both with reduced tumor growth and enhanced CD8+T cell recruitment into the tumor. These tumors contain a significant number of CXCR1,2 expressing myeloid-derived suppressor cells (MDSCs) that create an immune suppressive tumor microenvironment. Our preliminary data show that when CXCR2 is deleted in myeloid cells, MDSC recruitment to tumor is reduced and tumor growth is inhibited. Moreover, systemic delivery of a CXCR1,2 inhibitor reduced the growth of NRASmut melanoma in mice (p<0.02), and inducible deletion of CXCR2 in melanocytes blocks melanoma formation in the inducible BRAFV600E/PTEN-/- melanoma mouse model. These intriguing findings support prior studies indicating a role for CXCR2 inhibitors for treatment of melanoma. However, the mechanisms and generality of response to CXCR1,2 antagonism require further elucidation. Premise and Hypothesis: CXCR2 plays critical and pleotropic role in melanoma by promoting tumorigenesis and inducing an immunosuppressive tumor environment. Moreover, combined CDK4/6 and MDM2 inhibition significantly inhibits the growth of mouse and human NRASmut melanoma tumors. We hypothesize that co-inhibition of CXCR1,2, CDK4/6, and MDM2 in NRASmut melanoma with acquired resistance to ICI will inhibit tumor cell proliferation, induce tumor cell death, stimulate anti- tumor immunity, and potentially overcome the acquired resistance to ICI. We propose 3 specific aims. 1)To examine the ability of CDK4/6i plus MDM2i, combined with a CXCR1,2 antagonist treatment, or CXCR1,2 agonist alone, to enhance or restore ICI sensitivity for NRASmut melanoma tumors. 2) To determine the role of melanocyte-expressed CXCR2 in melanoma initiation. CXCR2 will be deleted coincident with induction of melanoma formation in mice and effects of this deletion on melanocyte apoptosis, senescence, differentiation and proliferation will be characterized. 3) To determine whether findings in mouse translate to human melanoma, changes in the tumor immune microenvironment (TIME) will be characterized over time in response to a CXCR1,2 antagonist (CXCR1,2i) with or without CDK4/6i+MDM2i followed by ICI therapy in humanized NRASmut patient-derived xenograft (PDX) models and human NRAS mut melanoma organoid co-cultures. These data will inform future clinical trials.